This invention relates generally to orthopedic prosthesis, particularly to those having a modular construction that is assembled from selected components and implanted during re-constructive arthroplastic surgery.
It is know that bone matter that is not stressed/loaded will atrophy and lose viability, a problem which currently persists in present orthopedic implants.
One approach others have taken is to provide a collar intended to correspond in shape and size to the prepared cavity of a proximal femur, ostensibly to offer rotary stability to the implanted device. For example, U.S. Pat. No. 4,790,852 to Noiles shows a modular hip prosthesis including a collar having a shape with a keyhole cross-section and terraces surrounding the outer surface perpendicular to the longitudinal axis of the collar. The collar has a tapered proximal-to-distal contour. However, the shape of the collar is instead dictated by the milling instrumentation used to prepare the proximal femoral cavity. According to the procedure, a surgeon resects the femoral head (ball) with an osteotome, thereby exposing the medial aspect of the cavity, then reams the intramedullary canal to make a space for the collar. The medial bone cavity is then milled to make it fit set criteria of the implant. U.S. Pat. No. 5,002,578 to Luman also has transverse terraces and a supposed cavity conforming cross-section. However, such terraces, like those of the above Noiles ""852 patent, do not counteract rotary motion of the prosthesis, but rather axial motion. U.S. Pat. No. 4,549,319 to Meyer has an external geometric pattern of elongated projections spaced circumferentially on a hip prosthesis. U.S. Pat. No. 4,624,673 to Meyer discusses a component for use in a prosthetic joint having a hollow tube with a closed end and an open end. The area of the external surface adjacent to the open end is at least twice the area of the external surface adjacent to the closed. A plurality of terraces, oriented orthogonal to a distal shaft, are on the external surface. A female part of self-locking taper is provided to connect the components. U.S. Pat. No. 4,846,839 to Noiles shows a modular hip with a collar having an oval cross-section and a terraced external surface, connecting with a stem. Products have been marketed generally based upon this approach, e.g., The SROM Hip, of Joint Medical Products, Inc.
The aforementioned patents seek to provide rotatory stability by circumferential contact between the prepared femoral cavity and an outer surface of a main body or collar member. This type of contact is actually tangential in nature. The use of an oval cross-sectional shape often does not allow the outer surface of the prosthesis collar to effectively engage the intact bone. In many cases, especially in revision patients, healthy bone can be problematic to uniformly contact, for example, in the lateral aspects of the proximal femur, where the greater trochanter is left largely undisturbed by the surgeon during a primary implantation. The lack of rotatory stability, needed for uniform stress on such healthy bone, can cause the intact tissue to weaken and possibly atrophy, unacceptably, at some point following the initial implantation of the prosthesis.
Others, including some of the above approaches, have further sought to achieve prosthesis-cavity conformance while providing various modular constructions and ways of connecting the components of the prosthesis.
For example, related U.S. Pat. Nos. 5,370,706 and 5,080,685 to Bolesky show a body member having a neck with a base defining a neck basal plane. A body member includes an upwardly and inwardly directed portion and a tapered longitudinal bore. A tapered connector engages a head member with the upwardly and inwardly directed portion. A problem experienced with this particular design is its limited strength, due to the location of the interconnecting components. Thus, the point of connection of the shaft must bear a load that is often too great, without being able to distribute that force. In U.S. Pat. No. 4,878,917 to Kranz, et al., there is shown a modular implant with a tensioning connector rod structurally designed to break when loaded a selected amount. U.S. Pat. No. 5,201,882 to Paxson discloses indicia for selecting the desired ante-version of a modular hip stem that is connected via tapered fittings to a unitary neck/body member, but the neck is not independently adjustable relative to the body. U.S. Pat. No. 5,725,592, issued to the present inventor, describes a modular hip prosthesis having a distal stem component that connects with a body component and a neck component both having tapered through-bores.
There is still a need for a modular prosthesis having independently adjustable components, and for a connector mechanism allowing the surgeon to visibly adjust them. Also, a prosthesis is needed, having an adjustable body with a geometry that incisively engages healthy bone, then securely locks with a stem component inter-operatively. The prior patents do not show a body shape capable of adjustably engaging healthy bone tissue in such a manner.
Other prior unitary implants have had various shapes, but the distal shaft and main body portions of these were not independently adjustable components, allowing surgeons to inter-operatively achieve optimal engagement of the body with healthy bone tissue.
Moreover, a need exists for a prosthesis that allows the surgeon to independently adjust the implant and realize the aim of engaging intact bone, as well, for an implant geometry that incises the intact bone and stresses it.
There is a further need for a modular implant that is infinitely adjustable, rotationally and axially, using a relatively simple array of components, allowing a surgeon flexibility while reducing the cost of carrying a large inventory of sizes. Such a need also exists while continuously delivering desired benefits, via the implant geometry, to a patient""s intact bone. A modular implant design is called for, whose components can be readily assembled inter-operatively and securely locked together by the surgeon to achieve these purposes.
According to the invention, there is provided a modular orthopedic prosthesis including a stem having one end with an elongated portion and another, longitudinally opposed end with a first means presenting a joint motion surface. The prosthesis has a body including a bore defining at least a first axis and having a polygonal cross-section with at least two angular corners adapted to bite or incise into the bone cavity of a patient to prevent rotation of the implanted prosthesis relative to the bone. Means are provided for connecting and locking the stem and body together in a fixed position.
In a preferred embodiment of this invention, the body has a first end with a generally triangular cross-section in the region of the first end and a generally round cross-section in the region of its second end, presenting a multi-axial wedge shape.
According to the invention, in another of its aspects, a modular orthopedic prosthesis is provided. The prosthesis comprises a stem component having one end with an elongated portion and another, longitudinally opposed end with a first means presenting a joint motion surface. A body component includes a bore defining at least a first axis. A radially expansible sleeve has opposed ends, one end received within the bore and the other end projecting outwardly from the bore, the sleeve adjustably connecting the stem and body together and locking them in a fixed position.
In a preferred embodiment of this invention, the joint motion surface is adjustably spaced from the body.
An advantage of the present invention is inventory and manufacturing cost savings, since its modularity allows for economy in manufacturing, as any number of well-known stem types may be selected, and also reduces the inventory of pieces needed on-hand for each surgical procedure.
Another advantage of the invention is enhanced clinical benefits, as the wedge shape offers enhanced rotatory stability of the prosthesis in use, particularly with revision surgery patients.
A further advantage of the invention is in providing a connector mechanism that is highly secure once fixed in place, meanwhile providing the surgeon inter-operative flexibility to axially and rotationally adjust the stem independently of the body.
A still further advantage of the invention gives a surgeon the capability to rotationally xe2x80x9cdialxe2x80x9d the body in order to have its triangular corners contact and xe2x80x9cbite intoxe2x80x9d or incise as much available healthy bone as possible. By evenly loading/stressing such intact bone, wherever found by the surgeon, particularly in revision patients, it is possible to avoid potential eventual atrophy of the bone tissue.
Other objects and advantages will become apparent to those skilled in the art by reference to the following Description and Drawings.